1. Field of the Invention.
The present invention relates to passivation of semiconductor surfaces, and, more particularly, to passivation of II-VI compound semiconductors such as Hg.sub.1-x Cd.sub.x Te.
2. Description of the Related Art.
Alloys of mercury telluride and cadmium telluride, generically denoted Hg.sub.1-x Cd.sub.x Te, are extensively employed as photosensitive semiconductors for infrared radiation detection. For example, Hg.sub..8 Cd.sub..2 Te has a bandgap of about 0.1 eV which corresponds to a photon wavelength of 12 .mu.m and Hg.sub..73 Cd.sub..27 Te has a bandgap of about 0.24 eV which corresponds to a photon wavelength of 5 .mu.m; these two wavelengths are in the two atmospheric windows of greatest interest for infrared detectors. In particular, p-n junction Hg.sub.1-x Cd.sub.x Te photodiode arrays have long been used (see, for example, Lorenze, U.S. Pat. No. 4,286,278), and extrinsic p type Hg.sub.1-x Cd.sub.x Te has potential application in infrared focal plane MIS detector arrays operating in the 10-12 .mu.m wavelength window. (Note that intrinsic p type Hg.sub.1-x Cd.sub.x Te, whose doping is presumably dominated by mercury vacancies, was recently found to have midgap recombination centers proportional in concentration to the shallow acceptors; see C.Jones et al, 3 J.Vac.Sci.Tech.A 131 (1985 ). These recombination centers shorten minority carrier lifetimes and are sources of recombination-generation noise; and thus extrinsic p type Hg.sub.1-x Cd.sub.x Te is preferred to intrinsic p type.) Such detectors are fabricated in large area Hg.sub.1-x Cd.sub.x Te which may be grown by LPE, MOCVD, MBE or bulk techniques and are operated typically at liquid nitrogen temperatures to limit background noise.
Passivation of Hg.sub.1-x Cd.sub.x Te prior to detector fabrication is necessary to avoid surface contamination by residues of various processing steps; such contamination affects the electrical characteristics of the detectors, for example, the photocarrier lifetime and stability. Analogous passivation of silicon for integrated circuits fabrication is typically achieved by growth of thermal oxides at temperatures about 1,000.degree. C.; however, thermal growth of oxides on Hg.sub.1-x Cd.sub.x Te is not feasible due to the out diffusion of mercury at even moderate temperatures. Consequently, passivation of Hg.sub.1-x Cd.sub.x Te by deposition of zinc sulfide or silicon dioxide has been used, but such passivation yields detectors that degrade (surface state density and accumulated surface charge vary and give unstable device characteristics) when subjected to temperatures over 70.degree. C. An improvement is passivation by anodic oxide: oxides of mercury, cadmium, and tellurium are grown on the surface of Hg.sub.1-x Cd.sub.x Te electrochemically in a KOH solution; see Catagnus, U.S. Pat. No. 3,977,018. Anodic oxide is also temperature sensitive and yields detectors that degrade at about 80.degree. C. Further, even extended storage at room temperature degrades such detectors.
Anodic sulfidization has also been proposed; see Teherani et al, U.S. Pat. No. 4,632,886 in which sulfides are grown on the surface of Hg.sub.1-x Cd.sub.x Te electrochemically in a Na.sub.2 S solution. These sulfides are stable up to 100.degree. C. and provide an improvement over anodic oxides. Similarly, selenides can be anodically grown. However, the anodic growth process requires complex electronic instrumentation and a means of bussing current between the instrumentation and the Hg.sub.1-x Cd.sub.x Te substrate submersed in the sulfide or selenide solution. Typically, a metal probe makes the electrical contact to the Hg.sub.1-x Cd.sub.x Te substrate, and this damages a portion of the substrate. Further, the resistive losses in the contacts, leads, and substrate yield a nonuniform sulfide or selenide passivation layer. These problems are magnified in large area substrates or a batch process with multiple substrates.
Copending U.S. patent application Ser. No. 824,897, filed Jan. 31, 1986, discloses a process of converting anodically-grown oxides to sulfides or selenides by immersion of oxide-coated substrates in sodium sulfide or selenide solutions. The oxides (typically HgTeO.sub.3, CdTeO.sub.3, and TeO.sub.2) are converted to sulfides or selenides (primarily CdS or CdSe) plus soluble complexes; the thickness of the resulting sulfide or selenide layer is typically about 40% of the thickness of the original oxide layer prior to conversion. But this process still requires the anodic oxide growth and its problems.
Thus it is a problem to provide a passivation for Hg.sub.1-x Cd.sub.x Te that avoids detector degradation at temperatures somewhat above room temperature and is uniform and simple to fabricate.